Blender Internal - colorless render (no materials)
Check your Render Layers settings - this includes an option to override the material for all objects in the scene :



If set, Blender will render all meshes with the specified material.

EDIT:

Newer versions of blender don't use layers, but "collections" so the override function is in the scene>View Layer section.



Additional info:

Where is the Layers panel in Blender 2.8?

2.8 - Is it possible to assign (override) a material to a whole collection?